Multiple format holographic CHMSLS

ABSTRACT

A hologram structure for use in a holographic center high mounted stoplight system for a vehicle. The hologram structure includes first and second hologram layers laminarly secured to each other, each having a plurality of holographic lenses formed therein, each holographic lens being configured to diffract light into a predetermined solid angular region.

This is a continuation application Ser. No. 08/153,732, filed Nov. 16, 1993, now abandoned as of Apr. 3, 1995.

BACKGROUND OF THE INVENTION

The disclosed invention is directed generally to holograms for center high mounted stoplights for vehicles, and more particularly to a hologram that efficiently meets the required brightness and angular coverage and to an exposure technique for recording such hologram.

Present federal regulations require center high mounted stoplights (CHMSLs) in addition to the standard stoplights mounted in the rear portion of an automobile. The high mounted stoplights are intended to maximize the visibility of the automobile braking indicators to drivers following the braking vehicle, and are commonly mounted on the rear window of an automobile.

High mounted stoplights have commonly been implemented as a standard lenticular lens, a red filter, an illuminating incandescent bulb, and a reflector enclosed in a housing that is typically secured adjacent the top or bottom of an automobile rear Window. However, the bulky housing partially obscures the rearward vision of the driver, and moreover imposes limitations on the design of the automobile.

Center high mounted stoplights have been also integrated into automobile body parts such as rear decks, spoilers, roofs, which to some degree substantially reduce or remove the rearward vision problem. However, such stoplights are complex and may impose limitations on the design of the automobile.

Holographic center high mounted stoplights have also been developed in order to efficiently meet the stoplight regulations. A consideration with the use of holograms for holographic center high mounted stoplights is the desire to achieve an output having a lenticulated appearance that is similar to that provided by conventional stoplights.

Another consideration with the use of holograms for holographic center high mounted stoplights is the complexity in meeting governmental luminous intensity and angular coverage requirements while utilizing a reconstruction light source of minimum power. The luminous intensity and angular coverage requirements generally include a quantitative aspect that defines luminous intensity over a defined central angular region, and a qualitative aspect that requires visibility over a horizontal angular region that is greater than the horizontal component of the defined central angular region. Essentially, the regulations require a brighter region generally in the center of the entire angular field of coverage.

SUMMARY OF THE INVENTION

It would therefore be an advantage to provide a holographic center high mounted stoplight that provides an output having an appearance that is similar to the lenticulated output of a conventional stoplight.

Another advantage would be to provide a holographic center high mounted stoplight that readily meets governmental luminous intensity and angular coverage requirements while utilizing a reconstruction light source of minimum power.

The foregoing and other advantages are provided by the invention in a holographic stoplight system that includes a light source for providing a reconstruction beam, a first hologram layer having a plurality of non-overlapping holographic lenses each of which is configured to diffract a portion of the reconstruction beam into a first predetermined angular field, and a second hologram layer having a plurality of non-overlapping holographic lenses each of which is configured to diffract a portion of the reconstruction beam into a second predetermined angular field, wherein the first and second hologram layers are laminarly attached to each other and wherein the portions of the reference beam diffracted by the first and second hologram layers comprise stoplight illumination.

BRIEF DESCRIPTION OF THE DRAWINGS

The advantages and features of the disclosed invention will readily be appreciated by persons skilled in the art from the following detailed description when read in conjunction with the drawing wherein:

FIG. 1 sets forth a schematic sectional view illustrating a holographic vehicle center high mounted stop light in accordance with the invention.

FIG. 2 sets forth a luminous intensity distribution graph depicting present vertical and horizontal requirements in candelas for center high mounted stoplights.

FIG. 3 schematically illustrates the organization of the cells of one of the holograms of the holographic center high mounted stop light of FIG. 1.

FIG. 4 schematically illustrates the organization of the cells of the other of the holograms of the holographic center high mounted stop light of FIG. 1.

FIGS. 5A and 5B schematically illustrate the playback characteristics of each of the hologram lenslets of the hologram of FIG. 3 in elevational view and in plan view.

FIGS. 6A and 6B schematically illustrate the playback characteristics of each of the hologram lenslets of the hologram of FIG. 4 in elevational view and in plan view.

FIG. 6C schematically illustrates the playback characteristics of an alternative configuration of each of the hologram lenslets of the hologram of FIG. 4 in plan view.

FIG. 7 sets forth by way of illustrative example an exposure system for constructing the CHMSL hologram of FIG. 3.

FIG. 8 sets forth by way of illustrative example a schematic illustration of a spherical lens array that is utilized in the exposure set up of FIG. 7 for construction of the hologram of FIG. 3.

FIG. 9 sets forth by way of illustrative example a schematic illustration of a mask that is utilized to define hologram cells in the exposure set up of FIG. 7 for construction of the hologram of FIG. 3.

FIG. 10 sets forth by way of illustrative example an exposure system for constructing the CHMSL hologram of FIG. 3.

FIG. 11 sets forth by way of illustrative example a schematic illustration of a holographic optical element array that is utilized in the exposure set up of FIG. 10 for construction of the hologram of FIG. 4.

FIG. 12 sets forth by way of illustrative example a schematic illustration of a mask that is utilized to define hologram cells in the exposure set up of FIG. 10 for construction of the hologram of FIG. 4.

FIG. 13 sets forth by way of illustrative example a schematic illustration of a masked holographic optical element array for use in the exposure set up of FIG. 10 for construction of the hologram of FIG. 4.

FIG. 14 forth by way of illustrative example a copy system for making hologram copies of hologram masters made pursuant to the exposure systems of FIGS. 7 and 10.

DETAILED DESCRIPTION OF THE DISCLOSURE

In the following detailed description and in the several figures of the drawing, like elements are identified with like reference numerals.

Referring now to FIG. 1, shown therein is a center high mounted stoplight system which includes a hologram assembly 20 and an illumination source 30. The hologram assembly 20, which includes first and second laminarly layered volume transmission holograms 31, 32 described further herein, is secured to the inside surface of an automobile rear window 211, and is substantially transparent to the rearward vision of the driver. Alternatively, the hologram assembly 20 can be secured to a separate substrate that is appropriately located near the rear window 211. The illumination source 30 is mounted forward of the hologram assembly 20 and out of the rearward viewing angle of the driver, for example, beneath the generally horizontal rear deck that is adjacent the lower portion of the rear window 211 of the automobile, and illuminates the volume holograms 31, 32 of the hologram assembly 20 with a reconstruction beam.

The volume holograms 31, 32 of the hologram assembly are constructed to provide, when illuminated by the illumination source 30, an appropriate composite image that is visible from behind the automobile over the appropriate vertical and horizontal viewing field. Since the following will discuss the volume holograms 31, 32 in terms of their angular coverage (i.e., the angular fields in which their light output can be seen), as well as their construction, angular fields will generally be with reference to the installed configuration of the holograms, for ease of reference. As used herein, the terms angular field and angular coverage will generally refer to all angular regions in space into which light is diffracted by a hologram cell thus refers to the angular region or regions in which the diffracted light is viewable. Since the angular coverage of a hologram defines the angular fields in which the light output of a hologram can be seen, it should be appreciated that the diffracted output of a hologram is confined to its angular coverage and some additional blurring. The holograms of the hologram assembly are configured to produce diffracted illumination that is visible from behind the vehicle in which they are installed, and thus their angular fields extend rearwardly away from the vehicle.

The illumination source 30 includes an incandescent lamp 15, such as a quartz halogen bulb, a reflector 17 that cooperates with the filament of the incandescent lamp 15 to provide a reconstruction beam to the hologram assembly 20, and a high pass filter 19 having a cutoff wavelength of about 600 nanometers (nm). By way of illustrative example, the reflector 17 comprises a parabolic reflector that cooperates with the filament of the incandescent lamp 15 to produce a near collimated reconstruction beam. As a result of the wavelength dependent sensitivity of human vision, the image provided by the hologram of the hologram assembly will be perceived as red, with the apparent peak intensity being somewhere between about 610 and 620 nm. The incandescent lamp 15 is connected to the brake light actuating circuit of the automobile so as to be energized when the brake pedal is depressed.

Referring now to FIG. 2, set forth therein is a graph showing the luminous intensity (in candelas) versus angular field requirements for a center high mounted stoplight for a central solid angular region defined as (a) 10 degrees up and 5 degrees down relative to a central horizontal plane H that passes through the center of the CHMSL, and (b) 10 degrees left and right relative to a central vertical plane V that passes through the center of the CHMSL and contains the longitudinal axis of the vehicle, as set forth in Federal Motor Vehicle Safety Standards No. 108 ("MVSS 108" herein). The numbers in the graph identify the minimum luminous intensities at those angular locations in candelas, and the entire angular field is subject to a 160 candela maximum at any given angular position.

In addition to the foregoing quantitative requirements, herein called the "central angular region requirements," there is a qualitative requirement that the stoplight be visible within the central horizontal plane over a horizontal angular field of 45 degrees on either side of the central vertical plane (i.e., two horizontal angular regions that are mirror images of each other). By way of example, this qualitative requirement can be interpreted as requiring a luminous intensity of about 1 candela. Such requirement as to visibility with the central horizontal plane over the range of 45 degrees on either side of the central vertical plane shall be called herein the wide angle horizontal requirement.

Referring now to FIG. 3, schematically depicted therein is the hologram 31 prior to lamination with the other hologram 32, with the surface shown being the surface that will face rearwardly when installed in a vehicle. The hologram comprises a rectilinear array or grid of non-overlapping contiguous hologram cells or facets C(i,j) of identical size, each hologram cell comprising a holographic lenslet that produces a diffracted output having a desired angular coverage that is the same for all facets C(i,j).

By way of reference, the top row of hologram cells C(1,j) in the figure comprises the top of the hologram 31 as it is positioned in the installed hologram assembly of the CHMSL system, whereby each row of holograms cells is generally aligned with the horizontal. Consistently with such orientation, the parallel edges of a cell that are generally aligned with horizontal when installed in the vehicle shall be called the top and bottom edges wherein the top edge is closest to the top of the figure and is closest to the top of the rear window when installed. Also, the parallel edges of a cell that are perpendicular to the top and bottom edges shall be referred to as the side edges. The cells C(i,j) are of identical size, and the width of a cell is measured along the top or bottom edge, while the height of a cell is measured along either of the side edges.

Referring now to FIG. 4, schematically depicted therein is the hologram 32 prior to lamination with the other hologram 31, with the surface shown being the surface that will face outside of vehicle when installed. The hologram comprises a rectilinear array or grid of non-overlapping contiguous hologram cells or facets F(i,j) of identical size, each hologram cell comprising a holographic lenslet that produces a diffracted output having a desired angular coverage that is substantially identical for all of the facets F(i,j).

By way of reference, the top row of hologram cells F(1,j) in the figure comprises the top of the hologram 32 as it is positioned in the installed hologram assembly of the CHMSL system, whereby each row of holograms cells is generally aligned with the horizontal. Consistently with such orientation, the parallel edges of a cell that are generally aligned with horizontal when installed in the vehicle shall be called the top and bottom edges wherein the top edge is closest to the top of the figure and is closest to the top of the rear window when installed. Also, the parallel edges of a cell that are perpendicular to the top and bottom edges shall be referred to as the side edges. The cells F(i,j) are of identical size, and the width of a cell is measured along the top or bottom edge, while the height of a cell is measured along either of the side edges.

In accordance with the invention, each of the facets or cells of one of the holograms 31, 32 is configured to diffract light into an angular field that comprises the central solid angular region depicted in FIG. 2, namely a solid angle having upper and lower limits of 10 degrees up and 5 degrees down relative to horizontal, and lateral limits of 10 degrees on either side of a vertical plane that is parallel to the longitudinal axis of the vehicle. In other words, the solid angular region is centered on a line that passes through the center of the cell, is about 2.5 degrees above horizontal, and is in a vertical plane that is parallel to the longitudinal axis of the vehicle. Each of the cells of the other hologram is configured to diffract light into an angular field that comprises a wide angular region that subtends a horizontal angle of 45 degrees on either side of the central vertical plane and a vertical angle that is quite small, for example in the order of 5 degrees above and below horizontal. Alternatively, each of the cells of such other hologram is configured to diffract light into an angular field that comprises two horizontal regions on either side of the central solid angular region.

For ease of reference, the hologram that diffracts light into the central solid angular region shall sometimes be called the narrow angle hologram while the other hologram shall sometimes be called the wide angle hologram. Each of the hologram cells of the narrow angle hologram produces an image of a masked point source and diffracts a portion of the reconstruction beam into the central solid angular region, while each of the hologram cells of the wide angle hologram produces an image of a masked vertical line source and diffracts a portion of the reconstruction beam into the wide horizontal field. In accordance with the invention, the cells C(i,j) of the first hologram 31 diffract light into respective substantially identical predetermined angular fields, and the cells F(i,j) of the second hologram 32 diffract light into respective substantially identical predetermined angular fields that are different from the predetermined angular fields of the cells C(i,j) of the hologram 31. By way of illustrative example, holograms 31, 32 have substantially identical arrays of hologram cells wherein the cell size is substantially identical and the arrangement of cells is substantially identical. By way of further illustrative example, the hologram cell arrays of the holograms 31, 32 are aligned when the holograms are laminarly secured to each other such that each cell in the hologram 31 has a corresponding cell in the hologram 32, and the boundaries of corresponding cells are aligned.

The number of cells in each of the holograms 31, 32 and the arrangement of cells in each of the holograms will depend on the overall shape and size of the entire format of each hologram and the selected cell size, wherein cell size is selected pursuant to consideration of factors such as appearance and ease of production. As discussed further herein, the invention contemplates that the ratio of cell width to foreshortened cell height is selected to satisfy the requirements of MVSS 108.

For the illustrative example wherein the first hologram 31 comprises the wide angle hologram and the second hologram 32 comprises the narrow hologram, each hologram cell F(i,j) of the second hologram is constructed such that upon reconstruction each cell F(i,j) produces the image of a masked point source that emits a diverging light beam into a solid angle having upper and lower limits of 10 degrees up and 5 degrees down relative to horizontal, and lateral limits of 10 degrees on either side of a vertical plane that is parallel to the longitudinal axis of the vehicle. Further for such illustrative example, each hologram cell C(i,j) of the first hologram 31 is constructed such that upon reconstruction each cell C(i,j) produces the image of a masked line source that emits a diverging light beam into a solid angle having upper and lower limits of about 5 degrees up and about 5 degrees down relative to horizontal, and lateral limits of 45 degrees on either side of a vertical plane that is parallel to the longitudinal axis of the vehicle. As more particularly described herein, each cell of the wide angle hologram can be alternatively constructed such that upon reconstruction each wide angle cell produces an image of a masked line source that emits diverging light into an angular field comprised of two peripheral angular regions on either side of the central solid angular region. For example, both peripheral solid angular regions have upper and lower limits of about 5 degrees up and about 5 degrees down relative to horizontal. One peripheral angular region is between 10 degrees and 45 degrees on one side of a vertical plane that is parallel to the longitudinal axis of the vehicle, while the other angular region is between 10 degrees and 45 degrees on the other side of a vertical plane. In other words, where a hologram cell of the wide angle hologram is constructed to diffract light into two solid angular regions, such regions are mirror images of each other about a vertical plane and are separated by a solid angular region having lateral limits of 10 degrees on either side of such vertical plane. Such two horizontal regions shall sometimes be called the peripheral horizontal angular regions.

It should be appreciated that while hologram angular coverage is discussed in terms of precise angles, the actual coverage will typically slightly exceed the constructed angular coverage due to blurring caused by a non-ideal reconstruction beam. Thus, to the extent that two hologram cells are constructed to have respective non-overlapping angular coverages that are contiguous without a gap in coverage, in practice there will be overlap in coverage due to blurring.

By way of illustrative examples, the hologram cells are configured such that the point and line source images produced by both holograms are in front of the hologram assembly (toward the front of the vehicle) or behind the hologram assembly (toward to a viewer behind the vehicle), or the cells of one hologram are configured such that the source images produced by one hologram are in front of (toward the front of the vehicle), while the source images produced by the other hologram are behind the hologram assembly (toward to a viewer).

The solid angular fields into which the hologram cells diffract light are controlled by the dimensions of the cell and the distance of the point or line source image from the corresponding cell. Since the cell size is the same for both holograms, and since the central solid angular region requirement of MVSS 108 specifies a vertical angular extent, the ratio of foreshortened cell height to cell width is selected to correspond to the ratio of vertical angular spread to horizontal angular spread of the central solid angular region, and the distance of the point source is selected to meet the specific central vertical and horizontal angular spread requirements. The distance of the line source is selected to meet the specific horizontal angular spread of the horizontal wide angle requirement of MVSS 108, and some vertical angular spread is provided by a less than ideal reconstruction source. Alternatively, the vertical angular spread of the line source image can be controlled by introducing slight amount of power in the vertical direction.

Referring in particular to FIGS. 5A and 5B, schematically set forth therein are an elevational side view and a top plan view of a hologram cell F, for the example wherein the second hologram 32 comprises the narrow angle, as oriented in an installed hologram assembly together with the diffracted light output beam P produced thereby. As shown in FIG. 5A, the upper and lower boundaries of the output beam P are determined by (1) a top plane TP defined by an apparent point source S and the top edge of the cell and (2) a bottom plane BP defined by the apparent point source and the lower edge of the cell. The top plane TP and the bottom plane BP are depicted by lines in FIG. 5A since such planes are orthogonal to the plane of the figure. The portions of the planes TP and BP shown by solid lines depict the playback beam limits while the portions of the such planes shown by broken lines represent the top and bottom limits of the virtual beam emanating from the apparent point source S. As shown in FIG. 5B, the lateral boundaries of the output beam P are determined by (1) a left side plane LP defined by the apparent point source S and the left edge of the cell and (2) a right side plane defined by the apparent point source S and the right edge of the cell. The left side plane LP and the right side plane RP are depicted by lines in FIG. 5B since such planes are orthogonal to the plane of the figure. As with the FIG. 5A, the portions of the planes LP and RP shown by solid lines represent the lateral limits of the playback beam while the portions of the planes LP and RP shown by broken lines depict the lateral limits of the virtual beam emanating from the apparent point source S.

From FIGS. 5A and 5B, it should be appreciated that, assuming the apparent point source S is centered laterally between the sides of a hologram cell F, the angular coverage of the output beam P and the orientation of such angular coverage relative to the hologram cell depends on the location of the apparent point source S and the ratio of (a) the width of the cell and (b) the subtended cell height (i.e., the height of the cell as projected onto a plane orthogonal to the center axis of the angular coverage), which are controlled by the construction of the hologram cell. In particular, each hologram cell F is exposed with a diverging object beam that emanates from a focus point and is masked at the hologram recording layer. The location of the object beam focus point, which defines the location of the apparent source S, and the ratio of the cell width to the subtended height are selected to provide the desired angular coverage for the cell.

Further as to the particular example wherein the second hologram 32 comprises the narrow hologram and the first hologram 31 comprises the wide angle 45 degree hologram, set forth in FIGS. 6A and 6B are an elevational side view and a top plan view of a hologram cell C of the hologram 31, as oriented in an installed hologram assembly together with the diffracted light output beam P produced thereby. As shown in FIG. 6A, the upper and lower boundaries of the output beam P are determined by (1) a top horizontal plane TP that contains the top edge of the cell and (2) a bottom plane BP that contains the lower edge of the cell. The top plane TP and the bottom plane BP are depicted by lines in FIG. 6A since such planes are orthogonal to the plane of the figure. The portions of the planes TP and BP shown by solid lines depict the playback beam limits while the portions of the such planes shown by broken lines represent the top and bottom limits of the virtual beam emanating from the apparent line source L. As shown in FIG. 6B, the lateral boundaries of the output beam P are determined by (1) a left side plane LP defined by the apparent line source L and the left edge of the cell and (2) a right side plane defined by the apparent line source L and the right edge of the cell. The left side plane LP and the right side plane RP are depicted by lines in FIG. 6B since such planes are orthogonal to the plane of the figure. As with the FIG. 6A, the portions of the planes LP and RP shown by solid lines represent the lateral limits of the playback beam while the portions of the planes LP and RP shown by broken lines depict the lateral limits of the virtual beam emanating from the apparent line source L.

For the particular example wherein the first hologram 31 comprises a wide angle hologram that diffracts light to peripheral solid angular regions on either side of the central solid angular region, set forth in FIG. 6C is a top plan view of a hologram cell C of the first hologram 31, as oriented in an installed hologram assembly together with the diffracted left and right output beams BL and BR produced thereby. The lateral boundaries of the left output beam BL is determined by (1) a left side plane LP defined by the apparent line source L and the left edge of the cell and (2) a plane LP' that is 10 degrees left of a vertical plane that passes through the center of the cell. The lateral boundaries of the right output beam BR are determined by (1) a right side plane RP defined by the apparent line source L and the right edge of the cell and (2) a plane RP' that is 10 degrees to the right of a vertical plane that passes through the center of the cell. The planes LP, LP', RP, RP' are depicted by lines in FIG. 6C since such planes are orthogonal to the plane of the figure. As in FIG. 6A, the portions of the planes LP, LP', RP, RP' shown by solid lines represent the lateral limits of the diffracted beams while the portions of the such planes shown by broken lines depict the lateral limits of the virtual beams emanating from the apparent line source L. As described further herein, the planes LP, LP', RP, RP' are defined by appropriate masking of the object beam utilized to construct the hologram cell C.

As to the foregoing particular example wherein the first hologram 31 is closest to the reconstruction source 30 and comprises the wide angle hologram, the wide angle hologram would have to be relatively inefficient to allow most of the reconstruction illumination to pass through to the narrow angle hologram which needs to provide a considerably brighter output. For example, the wide angle hologram could have an efficiency of about 30-40% for ideal reconstruction illumination while the narrow angle hologram could have an efficiency of at least 90% for ideal reconstruction illumination.

To the extent that the wide angle hologram is implemented with relatively high efficiency, the second hologram 32 should be implemented as the wide angle hologram and the first hologram should be implemented as the narrow angle hologram, and both holograms should have relatively high efficiencies of at least 90% for ideal reconstruction illumination. These conditions follow from the requirement that the diffracted light output of the narrow angle hologram must be considerably greater than the diffracted light output of the wide angle hologram. By making the narrow angle hologram relatively efficient and locating it closest to the reconstruction source, the narrow angle hologram can meet the luminous intensity requirements by diffracting most of the reconstruction illumination. Since most of the reconstruction illumination is diffracted by the narrow angle hologram, the reconstruction illumination available to the wide angle hologram is reduced considerably and it also must have a relatively high efficiency of at least 90% for ideal reconstruction illumination.

It should be pointed out that as to both of the foregoing implementations as to the sequence of the narrow angle and wide angle holograms, loss due to re-diffraction should be considered.

In the implementation wherein the wide angle hologram is closest to reconstruction source, only a small amount of diffracted light would be incident on the narrow angle hologram as result of the relatively low efficiency of the wide angle hologram for such implementation. Accordingly, only a relatively small amount of diffracted light would be re-diffracted by the narrow angle hologram, particularly since only the diffracted light within the narrow central angular region would be re-diffracted. To the extent that reduction of re-diffraction is desired, the cells of the wide angle hologram can be constructed to have a reduced angular overlap with the narrow angle hologram whereby the wide angle hologram diffracts most of its light into the peripheral horizontal regions on either side of the central angular region, for example as described above relative to the hologram cell of FIG. 6C.

In the implementation wherein the narrow angle hologram is closest to reconstruction source, a large amount of diffracted light within the narrow angle central region defined by MVSS 108 will be incident on the wide angle hologram, and to the extent that the wide angle hologram is implemented with hologram cells each of which diffracts light into a horizontal angular field that is 45 degrees on either side of a vertical plane that is parallel to the central vertical plane, such hologram cells of the wide angle hologram are capable of diffracting light that is within the portion of the horizontal angular field that overlaps with the central angular region. Accordingly, it may be beneficial to construct the hologram cells of the wide angle hologram to have a reduced angular overlap with the narrow angle hologram whereby the wide angle hologram diffracts most of its light into the horizontal regions on either side of the central region, as described above relative to the hologram cell of FIG. 6C. In other words, the wide angle hologram can be constructed to reduce the amount of light it diffracts into the central angular region, so as to reduce re-diffraction of the light diffracted by the narrow angle hologram.

As to the construction of the first and second holograms, all cells in a hologram can be exposed to diffract identically, or they can be exposed to have different diffraction characteristics, depending upon the specific requirements such as the curvature of the installed hologram assembly or non-collimation of the illumination source, Further, the cells in a hologram can be exposed one cell at a time or several cells at a time.

Referring now to FIG. 7, set forth therein by way of illustrative is an exposure set up for recording the central region hologram by a sequence of exposures wherein each exposure exposes a plurality of uniformly spaced non-contiguous cells in a row. The exposure set up includes a horizontal linear array 53 of plano-convex spherical lenses 53a, schematically depicted in FIG. 8 as viewed along a collimated beam CB1. The plano-convex spherical lenses have their curved surfaces toward the collimated beam CB1, as depicted in FIG. 7, and have a horizontal center-to-center spacing S1 that is determined by the size required for each of the plano-convex spherical lenses 53a to produce the desired vertical and horizontal angular spread of the cells of the central region hologram. Further, the center-to-center spacing S1 is equal to the center-to-center spacing of an integral number of adjacent hologram cells. The exposure set up further employs a film type mask 51, as schematically depicted in FIG. 4, that is opaque except for uniformly spaced transparent areas 51a of the same size as each of the hologram cells of the CHMSL holograms 31, 32 and aligned with the plano-convex spherical lenses 53a so as to have the same center-to-center spacing S1 as the plano-convex lenses 53a. The number of transparent areas is the same as the number of plano-convex lenses 53a.

As more particularly shown in FIG. 7, a holographic recording assembly 60 is disposed beneath an exposure mask assembly 50. The holographic recording assembly 60 is comprised of a glass substrate 63, a holographic recording layer 61 disposed on the glass substrate 63, a thin cover layer 65 disposed over the holographic recording layer 61, and a light absorbing layer 71 adjacent the bottom of the glass substrate 63 and index matched thereto by layer of index matching fluid 73. The exposure mask assembly 50 is comprised of the mask 51 and a mask supporting substrate 55. The hologram recording assembly 60 and the exposure mask assembly 50 are oriented with the exposure mask 51 and the thin cover layer 65 facing each other and separated by a very thin layer of index matching fluid 57.

By way of illustrative example, the holographic recording assembly 60 is movable, for example by a computer driven motor, such that hologram forming cells that correspond to the hologram cells of the CHMSL hologram can be selectively positioned beneath the mask transparent areas 51a for exposure. In order to limit exposure to the hologram forming cells uncovered by the mask transparent areas, the mask and the mask substrate are secured in the opening of an opaque top cover of a conventional oil gate trough 69, wherein the top cover is sufficiently large to ensure that only the portions of the hologram recording layer beneath the mask transparent areas 51a will be exposed to construction illumination for all exposure positions of the hologram recording layer.

The object beams OB are produced by portions of a collimated beam CB1 which pass through the spherical lenses 53, converge down to respective focus points F, and then diverge to provide diverging object beams OB for the hologram forming cells of the hologram recording layer uncovered by the mask transparent areas 51a. Based on the geometrical considerations discussed above relative to FIGS. 6A and 6B, the size of the hologram forming cells and the position of the spherical lens array relative to the mask transparent areas is selected so as to produce the desired angular coverage in the vertical and horizontal directions for the resulting hologram cells, taking into account foreshortening caused by the tilt of the installed hologram.

The reference beams for exposure comprise portions of a collimated beam CB2, whose angle is based on an illustrative example of a CHMSL that is illuminated from below as illustrated in FIG. 1. The top of the CHMSL hologram corresponds to the left edge of the hologram recording layer that is perpendicular to the plane of the figure.

Exposure of hologram forming cells uncovered by the mask transparent areas 51a is accomplished by positioning the hologram recording assembly to locate the areas of the hologram recording layer corresponding to the hologram cells beneath the mask transparent areas and illuminating the unmasked cells with construction illumination via the plano-spherical lens array and the collimated beam CB2.

By way of illustrative example, the hologram cell rows are exposed in sequence, and within each row cell, subsets of the set of hologram cells that ultimately form the row can be exposed in a sequence. Thus, for the particular example wherein the spacing between mask transparent areas is three cells, for each row i the hologram recording assembly would be moved to uncover a first cell subset comprised of the cells C(i,j) wherein j is equal to 1, 4, 7, 10, etc. After the hologram recording assembly has settled, such first cell subset would be exposed with construction illumination. The hologram recording assembly would then be moved to uncover a second cell subset comprised of the cells C(i,j) wherein j is 2, 5, 8, 11, etc. After the hologram recording assembly has settled, such second cell subset would be exposed with construction illumination. The procedure of indexing the hologram recording assembly by one cell along a row, and exposing the uncovered cells with construction illumination would be repeated as required to expose all of the cells in a given row. Depending upon the number of cells in a row, the number of transparent areas in the mask, and the spacing between mask transparent areas, it should be appreciated that in one or more of the later indexed positions, the number of cells uncovered can be less than the number of cells uncovered in the first indexed position.

In the foregoing procedure wherein the hologram recording layer is moved relative to the mask, the object beam forming lenses are stationary, and the reference beams comprise collimated beams, each hologram forming cell is exposed with substantially the same reference beam incidence angle where such reference beam angle is formed between the plane of the cell and a line that passes through the center of the cell and is parallel to the central axis of the collimated beam CB2. To the extent that a non-collimated reference beam shape is utilized, the reference beam angle would be the same for all cells but the playback beam angle for a corresponding non-collimated playback beam shape will not be the same for all cells, which will result in reduced brightness if constructed with respective reference beams having substantially the same incidence angle. In order to more accurately emulate the playback source, the reference beam incidence angle can be changed for each row of cells such that the reference beam incidence angle for each cell more closely approximates the playback incidence angle intended for that cell, for example as disclosed in commonly assigned copending application U.S. Ser. No. 07/995,117, filed Dec. 22, 1992, by Smith et al., entitled "MODIFIED TECHNIQUE FOR FABRICATING LOW NOISE FLOODLIT CHMSL HOLOGRAMS", incorporated herein by reference.

Referring now to FIG. 10, set forth therein by way of illustrative example is an exposure set up for recording the wide angle hologram by a sequence of exposures wherein each exposure exposes a plurality of uniformly spaced non-contiguous cells in a row. The exposure system of FIG. 10 is similar to the exposure system of FIG. 7, and like elements therein are designated with like reference numerals. The differences in the exposure system include a linear array 153 of holographic optical elements 153a that are reconstructed by a collimated reconstruction beam RB, and a film type mask 151 that is opaque except for transparent areas 151a as shown in FIG. 12. The reconstruction beam incident surfaces of the holographic optical elements are shown in FIG. 11. Each of the holographic optical elements 153a comprises a simple cylinder lens (i.e., its output emulates the output of a refractive cylindrical lens), for example, whose cylinder axis is parallel to the plane of FIG. 10. The output of each holographic optical element 153a comprises a beam that converges to a line focus L which is parallel to the plane of the figure, and then diverges, such that the light incident on the hologram recording layer is diverging along an axis that is perpendicular to the plane of the figure. The holographic optical elements are positioned sufficiently close to the mask transparent areas 151a to achieve the horizontal angular spread required of the wide angle hologram. The holographic optical elements 153a and the mask transparent areas 151a are aligned with each other with a horizontal center-to-center spacing S2 that is determined by the amount of spacing required for each of the holographic optical elements 153a to produce the desired vertical and horizontal angular spread at areas uncovered by the mask transparent areas without intersection of the beams at the hologram areas uncovered by the mask transparent areas. In other words, the holographic optical elements 153a must be separated from each other such that their outputs do not overlap at the hologram areas uncovered by the mask transparent areas 151a. Further, the center-to-center spacing S2 is equal to the center-to-center spacing of an integral number of adjacent hologram cells, and can be different from the horizontal center-to-center spacing S1 of the transparent areas 51a of the mask 51 of FIG. 4.

By way of illustrative example, the hologram cell rows of wide angle hologram made pursuant to the exposure set up of FIG. 10 are exposed in an indexed manner as described above with respect to the exposure set up of FIG. 7.

Further in accordance with the invention, each of the holographic cells of the wide angle hologram can be configured to produce a beam that provides the required 90 degree angular spread in the installed horizontal direction, as well as a few degrees angular spread in the vertical direction, for example by being constructed with an object beam that comprises the output of a cylindrical lens that is illuminated with a converging beam. This results in a holographic optical element that emulates a cylindrical lens with a slight amount of spherical power.

It should be appreciated that the wide angle hologram of the invention can be constructed using a linear array of cylindrical lenses substituted for the holographic optical elements of the exposure set up of FIG. 8.

To the extent that it is desired to reduce the angular overlap of the wide angle hologram with respect to the narrow angle hologram, the wide angle hologram can be constructed so as to significantly reduce the light it diffracts into central solid angular region. In particular, respective masks 53b can be attached to the holographic optical elements 53a as shown in FIG. 13 such that object beam illumination within the angular region corresponding to the central solid region does not reach the holographic recording layer. Each mask 153b extends the vertical extent of holographic optical element to which it is attached and has a lateral extent that is selected so as to block object beam illumination from a desired angular region, for example the angular region that extends horizontally between 10 degrees on either side of center as defined by MVSS 108. In this manner, the wide angle hologram has little angular overlap with the narrow angle hologram, and loss due to re-diffraction is reduced.

Further in accordance with the invention, a narrow angle hologram master and a wide angle hologram master can be made with the exposure systems of FIGS. 7 and 10, respectively, and such masters are then utilized to make the holograms 31, 32 of the holographic center high mounted stoplight of FIG. 1. The narrow angle hologram master and the wide angle hologram master can be copied separately to make separate copies of each, or they can be laminated together and copied to make a composite hologram. In either case, the efficiencies of the narrow angle hologram master and the wide angle hologram master made pursuant to the exposure systems of FIGS. 7 and 10 need to the selected such that the hologram copies provide the desired efficiencies.

Referring in particular to FIG. 14, schematically depicted therein is a copy system for making hologram copies from hologram masters made pursuant to the exposure systems of FIGS. 7 and 10. The copy system includes a master hologram 111, an adjacent holographic recording layer 113 that is located at a minimum distance from the master hologram, and an intervening thin layer of index matching fluid 115. The master hologram is supported by a glass substrate 117, while the holographic recording layer 113 is supported by an opaque backing layer 119. In use, a copy reference beam RB that emulates the reference beam utilized to construct the master hologram 111 is directed to the glass substrate at an angle that produces the proper beam angle for reconstruction of the hologram master 111. The copy reference beam passes through the glass substrate 117, and is partially diffracted by the master hologram 111. The diffracted and undiffracted light from the master hologram interfere in the holographic recording layer to make the hologram copy.

For maximum efficiency for a narrow angle hologram copy, the narrow angle hologram utilized as the narrow angle hologram master should have an efficiency of 50% so as to provide a 1 to 1 beam ratio for the example of a holographic recording layer comprising DuPont HRF 600. For a wide angle hologram copy having an efficiency in the range of about 30% to 40%, for the implementation wherein the wide angle hologram is closest to the reconstruction source 30, the wide angle hologram utilized as the wide angle hologram master should have an efficiency in the range of about 10% to 20% for the example of a DuPont HRF 600 photopolymer holographic recording layer. For a wide angle hologram copy having an efficiency of at least 90%, for the implementation wherein the narrow angle hologram is closest to the reconstruction source 30, the wide angle hologram utilized as the wide angle hologram master should have an efficiency in the range of about 50% for the example of a DuPont HRF 600 photopolymer holographic recording layer.

As discussed previously, the narrow angle hologram master and the wide angle hologram master can be copied separately to make separate copies of each, or they can be laminated together and copied to make a composite hologram copy that would be utilized in place of the first and second holograms 31, 32. The copy system of FIG. 14 can be utilized to produce a composite hologram copy by utilizing a laminate of a narrow angle hologram master and a wide angle hologram master as the hologram master 111. In the resulting copy hologram the wide angle holograms cells and the narrow angle hologram cells are superimposed.

In making a composite copy, the relative efficiencies of the narrow angle and wide angle hologram masters must be properly selected since light diffracted light from one hologram master can interfere with diffracted light from the other hologram master to produce cross-modulation noise holograms. This problem can be alleviated by reducing the diffracted outputs of both hologram masters that comprise object beams in the construction of the composite hologram copy. For example, a narrow angle hologram master efficiency of about 25% and a wide angle hologram master efficiency of about 5% for a DuPont HRF 600 photopolymer holographic recording layer sufficiently reduces the cross-modulation noise holograms while minimizing loss in the efficiencies of the desired diffraction directions of the copy.

While the hologram assembly of the invention has been described in the foregoing in the context of holograms that provide orthoscopic images for purposes of illustrative example, the invention also contemplates holograms that provide pseudoscopic images. For example, one or both of the holograms 31, 32 of the hologram assembly can be rotated 180 degrees about a horizontal axis such that the original top of the hologram is at the bottom and the hologram surface that was originally toward the inside of the vehicle is now toward the outside of the vehicle. Each of the cells of a hologram rotated in such manner would produce an image of a point source or a line source that is located behind the hologram (i.e., towards the rear of the vehicle), with the horizontal and vertical spread being determined as if a converging beam were masked by the boundaries of the cell.

The foregoing has been a disclosure of a holographic center high mounted stoplight that meets governmentally required luminous intensity and angular coverage requirements while minimizing the power required. The hologram structure of the holographic center high mounted stoplight of the invention can be configured to produce a lenticulated appearance similar to that of conventional stoplights, and is amenable to efficient production by separate copying of respective hologram layers to respective hologram copy layers, or by copying of a plurality of hologram layers to a single layer hologram copy.

Although the foregoing has been a description and illustration of specific embodiments of the invention, various modifications and changes thereto can be made by persons skilled in the art without departing from the scope and spirit of the invention as defined by the following claims. 

What is claimed is:
 1. A holographic center high mounted stoplight system for a vehicle, comprising:a light source for providing a reconstruction beam; a first array of non-overlapping transmission holographic lenses, each of said holographic lenses being configured to diffract a portion of the reconstruction beam into a first predetermined angular field that extends rearwardly relative to the vehicle, said first predetermined angular field comprising a central angular region having vertical and horizontal extent; and a second array of non-overlapping transmission holographic lenses superimposed with said first array of non-overlapping transmission holographic lenses, each of said holographic lenses in said second holographic layer configured to diffract a portion of said reconstruction beam into a second predetermined angular field that extends rearwardly relative to the vehicle, said second predetermined angular field comprising first and second peripheral angular regions on either side of said central angular region; whereby said portions of said reconstruction beam that are diffracted by said holographic lenses of said first and second holograms form stoplight illumination.
 2. The holographic center high mounted stoplight system of claim 1 wherein said first array of transmission holographic lenses is formed in a first hologram layer, and wherein said second array of transmission holographic lenses is formed in a second hologram layer, said first and second hologram layers being laminarly attached to each other.
 3. The holographic center high mounted stoplight system of claim 2 wherein each of said transmission holographic lenses of said first array produces an image of a point source, and wherein each of said transmission holographic lenses of said second array produces an image of a line source.
 4. The holographic center high mounted stoplight system of claim 3 wherein said image of a point source and said image of a line source are displaced away from one side of said hologram layers.
 5. The holographic center high mounted stoplight system of claim 3 wherein said image of a point source is displaced away from one side of said hologram layers, and wherein said image of a line source is displaced from another side of said hologram layers.
 6. The holographic center high mounted stoplight system of claim 1 wherein said first array of transmission holographic lenses and said second array of transmission holographic lenses are formed in a single hologram layer.
 7. The holographic center high mounted stoplight system of claim 6 wherein each of said transmission holographic lenses of said first array produces an image of a point source, and wherein each of said transmission holographic lenses of said second array produces an image of a line source.
 8. The holographic center high mounted stoplight system of claim 7 wherein said image of a point source and said image of a line source are displaced away from one side of said single hologram layer.
 9. The holographic center high mounted stoplight system of claim 7 wherein said image of a point source is displaced away from one side of said single hologram layer, and wherein said image of a line source is displaced from another side of said single hologram layer.
 10. The holographic center high mounted stoplight system of claim 1 wherein said transmission holographic lenses of said first array and said transmission holographic lenses of said second array are substantially identical in shape and arrangement.
 11. The holographic center high mounted stoplight system of claim 10 wherein said first array of transmission holographic lenses is formed in a first hologram layer, and wherein said second array of transmission holographic lenses is formed in a second hologram layer, said first and second hologram layers being laminarly attached to each other.
 12. The holographic center high mounted stoplight system of claim 11 wherein each of said transmission holographic lenses of said first array produces an image of a point source, and wherein each of said transmission holographic lenses of said second array produces an image of a line source.
 13. The holographic center high mounted stoplight system of claim 10 wherein said first array of transmission holographic lenses and said second array of transmission holographic lenses are formed in a single hologram layer.
 14. The holographic center high mounted stoplight system of claim 13 wherein each of said transmission holographic lenses of said first array produces an image of a point source, and wherein each of said transmission holographic lenses of said second array produces an image of a line source.
 15. A hologram exposure system comprising:an array of holographic optical elements each configured to produce a beam having a predetermined angular field; a reconstruction beam for illuminating said array of holographic optical elements; a plurality of masks respectively disposed on respective ones of said holographic optical elements for masking said holographic optical elements such that each holographic optical element produces an output that includes a plurality of beams; a construction beam; and a hologram recording layer illuminated by the output beams of said holographic optical elements and said construction beam for recording a hologram therein. 